A photocatalyst is a functional material that exhibits its catalytic action when it absorbs light, and titania (TiO2) that exhibits activity when exposed to ultraviolet light is known as a representative photocatalyst. In addition, titania having a peroxy bond and a composite compound that includes titania and silica bonded to each other and has a peroxy bond (hereinafter referred to as “titania-silica”) are known as photocatalysts having not only an improved photocatalytic function but also additional functions (see, for example, Patent Literature 1). In such titania-silica, the photocatalytic function is exerted mainly by the titania structure portion, a hydrophilic function is exerted mainly by the silica structure portion, and a visible light absorption function is exerted mainly by the peroxy bond structure portion.
Such a photocatalyst with its catalytic action has a function of decomposing soil such as organic matter. Therefore, soil-resistant glass that can be prevented from being soiled over a long time can be produced by coating the glass surface with the photocatalyst.
In general, as a conventional method for coating a glass surface with a photocatalyst, the glass surface is coated with a solution comprising the photocatalyst, and the applied solution is subjected to heat treatment to form the coating of the photocatalyst. In the conventional method, since the crystallization temperature of titania is at least 300° C. or higher, the heating temperature when the coating is formed has been set to 300 to 500° C. to bake the coating onto the glass surface.
However, the conventional method requires an apparatus for heating to high temperatures of 300° C. or higher and a separate apparatus for cooling and has the problem in that manufacturing cost is high because the amount of heat is large. In addition, since the maximum use temperature of, for example, float glass used for a pane is approximately 380° C., the heat treatment may cause a reduction in activity and other adverse effects on the glass itself due to, for example, permeation of titania. Therefore, it is desirable to lower the heating temperature. However, it is concerned that, when the heating temperature is lowered, the adhesion and hardness of the coating film become insufficient. With the conventional photocatalyst solution, it is difficult to overcome all the foregoing problems, and therefore there are still many hurdles to overcome to establish the integrated production of a high-activity photocatalyst-coated glass plate in a factory.